The development of an in vivo system permitting the control of virogene marker expression by the manipulation of the host's endocrine and immunological balances now provides a model to comprehensively examine the relevance of viral information in tumorigenesis in host/graft (maternal/fetal) interactions and in embryological development. Efforts will be concentrated on physiological and immunological mechanisms as they interact with host genetics and immunocompetence in the induction and regulation of virus expression.